Medicine

If you like history then stick with it; it will in no way put you at a disadvantage as long as you have chemistry and biology. I think Cambridge puts emphasis on having 3 sciences, UCL slightly prefers a contrasting non-science subject- at most other unis your third a level doesn't matter as long as it is 'rigorous' and 'academic' (which history is).

Better picking a non-science subject you can get an A in than getting a B in maths.